Plasma cortisol, oestrone sulphate and progestagens were measured in 22 stressed pregnant mares (gestation length 17-336 days) as indicators of fetal viability. Mares were bled every 12 h from time of admission, and plasma was stored at -70 degrees C until assayed. Four normal mares were bled twice weekly from Day 270 to parturition to provide baseline endocrine data. Cortisol and progestagen concentrations were measured by radioimmunoassay and oestrone sulphate was measured by enzyme immunoassay. Mares were grouped according to clinical diagnosis: surgical colic (Group 1, n = 11), medical colic (Group 2, n = 7), and uterine torsion (Group 3, n = 4). Of the 16 mares in Groups 1 and 2 that survived to discharge, 12 mares foaled normally and 4 aborted, 3 during hospitalization. Following surgical treatment of uterine torsion, 2 mares aborted and 2 mares carried foals to term. Plasma cortisol was greater than 30 ng/ml in 19 of the 22 stressed mares at presentation and was less than 30 ng/ml in normal mares at all collections. Cortisol concentrations remained elevated in mares during post admission complications. The mean cortisol concentration of mares with colic that subsequently aborted was higher at presentation, but not statistically different, than levels of mares that did not abort (135 +/- 35 ng/ml and 83 +/- 19 ng/ml, respectively; mean +/- s.e.m.). Progestagen concentrations in normal mares ranged from 2-25 ng/ml.(ABSTRACT TRUNCATED AT 250 WORDS)